Summary:
NuoC is part of the connecting fragment of NADH dehydrogenase I [Leif95].

Unlike in other bacteria, which contain two separate genes encoding the NuoC and NuoD subunits, the nuoC gene of E. coli K-12 encodes a fused version of these subunits [Braun98a]. NuoC is the only subunit of the peripheral arm that does not contain a cofactor. This subunit was predicted to function as the proton channel [Friedrich98a]. NuoC interacts with FliG and FliM, components of the flagellar switch-motor complex [Zarbiv12].

Mutagenesis of two conserved histidine residues, H224 and H228, only has a modest effect on ubiquinone reductase activity of NDH-1. An R274A mutant leads to a significant loss of signal from the N2 4Fe-4S cluster as well as from a second fast-relaxing 4Fe-4S cluster [Belevich07]. Mutagenesis of several highly conserved amino acid residues in the NuoC domain showed that certain Glu and Asp residues are required for energy transduction of NDH-1, as well as for assembly of the enzyme [CastroGuerrero10].

Null mutants of all individual nuo genes have a growth defect under aerobic conditions in rich medium [Erhardt12].

nuoC is one of a network of genes believed to play a role in promoting the stress-induced mutagenesis (SIM) response of E. coli K-12 [Al12].

Summary:
NADH:ubiquinone oxidoreductase I (NDH-1) is an NADH dehydrogenase that catalyzes the transfer of electrons from NADH to the quinone pool in the cytoplasmic membrane and is able to generate a proton electrochemical gradient. It is part of both the aerobic and anaerobic respiratory chain of the cell. The study of this enzyme is of great interest, because it is considered to be a structurally minimal form of a proton-pumping NADH:ubiquinone oxidoreductase and serves as a model for the more complex mitochondrial enzyme.

NDH-1 is one of two distinct NADH dehydrogenases in E. coli. In contrast to NDH-2 (encoded by ndh), NDH-1-catalyzed electron flow from NADH to ubiquinone generates an electrochemical gradient. Depending on the strain, NDH-2 utilizes NADH exclusively, while NDH-1 can utilize both NADH and d-NADH, which enables specific assays of the enzyme [Matsushita87, Hayashi89, Calhoun93]

Crystal structures of the membrane domain of NDH-1 have been solved at 3.9 Å resolution [Efremov10] and later at 3 Å resolution [Efremov11]. A plausible mechanism of electron transfer and its coupling to proton translocation has been deduced from this crystal structure and that of the Thermus thermophilus enzyme [Sazanov07, Efremov10]. Proton translocation may be induced by movement of the long amphipathic α-helix of the NuoL subunit that is aligned parallel to the membrane [Efremov10]. This model is discussed in a comment by [Ohnishi10]. The exact number of protons translocated across the membrane remains unknown; the H+/e- stoichiometry is at least 1.5 [Bogachev96]. Recent experiments argue for at least two coupling sites for proton translocation, with NuoL being essential for the translocation of 2H+/2e- [Steimle11]. A crystal structure of the membrane component at higher resolution has allowed identification of possible proton translocation pathways and argues for a purely conformation-driven pathway of proton translocation [Efremov11]. Zn2+ inhibits Ndh-1, possibly by blocking the entry or exit of a proton translocation pathway [Schulte14].

Based on a stoichiometry of 4 H+ translocated per NADH oxidized (2e-), a mixed model for proton translocation using both direct (redox-driven) and indirect (conformation-driven) mechanisms for proton pumping has been presented [Treberg11]. However, a lower ratio of 3H+/2e- has recently been proposed [Wikstrom12]. The Ndh-1 catalytic cycle has been followed in real time, revealing an essentially biphasic reaction [Belevich14].

The purified enzyme can be separated into three components: a soluble fragment composed of the NuoE, F and G subunits which catalyzes the oxidation of NADH, representing the electron input part of the enzyme [Braun98a]; an amphipathic connecting fragment composed of the NuoB, CD and I subunits; and a hydrophobic membrane fragment composed of the NuoA, H, J, K, L, M and N subunits [Leif95]. The soluble subunits contain all iron-sulfur clusters and the FMN cofactor; the redox properties of those cofactors have been studied [Euro08], and their intrinsic redox potential was modeled [Medvedev10]. Electron transfer from NADH via FMN to the iron-sulfur centers has been measured in real time [Verkhovskaya08]. Results from crosslinking analysis suggest that the ubiquinone-binding site of the enzyme is located on the membrane subunit NuoM [Gong03a], but it has also been modeled to the interface between NuoB and NuoCD based on its location in the T. thermophilus enzyme [Baranova07]. Site-directed spin labeling is being used for localization of the ubiquinone binding site [Pohl10]. There may be two ubiquinone binding sites [Verkhovsky12], and NDH-1 purified using a new procedure contained two molecules of ubiquinone per complex [Narayanan13]. A tightly bound ubiquinone found by [Verkhovskaya14] has a very low midpoint potential of < -300 mV, while two quinone radicals found by [Hielscher13] had midpoint potentials of -37 and -235 mV. The NuoJ [Kao05a], NuoK [Kervinen04, Kao05b], NuoM [TorresBacete07] and NuoN [Amarneh03] subunits are implicated in the ability to generate an electrochemical gradient. Molecular dynamics simulations of the membrane domain have established a possible coupling mechanism for energy transduction within NDH-1 [Kaila14].

Three-dimensional reconstruction and 2-D crystals of the NDH-1 complex based on cryo-electron microscopy showed an L-shaped form with an integral membrane and a peripheral arm [Guenebaut98, Holt03]. A model of the spatial arrangement of the subunits and the possible functional mechanism of proton pumping has been proposed [Holt03]. Under low ionic strength conditions, the complex appears to adopt a horseshoe-like conformation [Bottcher02]. Cryo-electron microscopy of the membrane domain allowed calculation of a projection structure at 8 Å resolution [Baranova07]; later, a cryo-EM 3D structure of the intact NDH-1 complex was obtained [Morgan08]. Binding of NADH induces a conformational change in both the membrane and peripheral arm of NDH-1 [Mamedova04, Pohl08]. A mechanism by which the redox reaction of the N2 Fe-S cluster induces a conformational change that may lead to proton translocation has been proposed [Friedrich10].

Heterooligomers of NDH-1 and NDH-2 have been identified by electrophoresis and sucrose gradient centrifugation suggestive of a supramolecular organisation in the membrane [Sousa11].

NDH-1 is required for the anaerobic respiration of NADH using fumarate or DMSO as the terminal electron acceptors, thus implying that the enzyme can transfer electrons to menaquinone [Tran97]. The comparative energy efficiency of utilization of the various components of the aerobic respiratory chain has been examined [Calhoun93a, Unden97].

Stolpe and Friedrich [Stolpe04] showed that NDH-1 is primarily an electrogenic proton pump which may have secondary Na+/H+ antiport activity. However, contrary to the generally accepted view, Steuber et al. [Steuber00] suggested that NDH-1 functions primarily as a Na+ pump, a function that can be conveyed by a truncated form of the NuoL subunit alone [Steuber03, Gemperli07].

NDH-1 produces reactive oxygen species, mainly in the form of H2O2, at the NADH dehydrogenase active site, involving the FMN cofactor [Esterhazy08]. NADH-dependent production of hydrogen peroxide is increased in a NuoF E95Q mutant [Knuuti13]. The rate of O2 reduction is dependent on the NAD+/NADH ratio [Esterhazy08].

Purified NDH-1 is activated by detergent and phospholipids [Sinegina05, Stolpe04]. A tightly bound metal, most likely Ca2+, is required for activity [Verkhovskaya11].

Mutants lacking NDH-1 can not compete with wild type in stationary phase [Zambrano93]. Expression of the nuo operon is regulated by oxygen, nitrate, fumarate, and other factors including C4 dicarboxylates [Bongaerts95, Tran97]. Transcription and activity of aerobic respiratory chain components in the different phases of aerobic growth have been measured [Sousa12].